OCR Text |
Show ( X 0 2 = 0.21, X N 2 = 0.79) or oxygen-deficient air ( X Q 2 = 0.15, X N j = 0.85). The reaction mechanism used was G R I version 2.1119 and a nominal strain rate of 100 s'1 was chosen. Figure 5 shows the mass fractions of major species, NO and other nitrogen containing species as a function of the local mixture fraction, / for combustion in air. The mixture fractions were calculated using the method of Starner et al.20 The maximum temperature of the flame (T-2035 K) coincides with the stoichiometric mixture fraction (fs = 0.074) as does the peak in N O mass fraction, indicating its source to be mainly from fuel-N or prompt routes. L The mass production rate of N O in the flame can be computed from J M N0 co N0 dx . o This results in a value of 6.8 x 10"6 g cm"3 s"1 for combustion in air and 4.0 x 10"6 g cm"3 s"1 for combustion in oxygen-deficient air (where T ^ -1775 K), a difference of approximately 4 1 %. To assess the contribution to NO production of fuel-nitrogen alone, the prompt and thermal NO formation pathways were switched off. This resulted in only a 4 % decrease in N O production in air and a 1 6 % reduction in N O using oxygen-deficient air, thus indicating that fuel-N contributions to total N O production are 9 6 % and 8 4 % respectively. Figure 6 shows the net rate of N O production as a function of mixture fraction for both combustion in air and oxygen-deficient air and it is clearly seen that fuel-N is the major source of N O . In each case, the maximum production rate occurs at the stoichiometric mixture fraction. A more realistic representation of NO production is to relate the mass of nitrogen in NO produced to the mass of nitrogen in the original fuel that is consumed via (L \ II JMN<6NOdx I \\MN <QHCN+MN <6m3)dx. vo // 0 This yields the fact that for the volatiles combustion in air, -83% of the fuel nitrogen consumed ends up in N O . A similar percentage was obtained for the case with oxygen deficient air. The output calculations of the code are processed to the format accepted by the CFD code. The model tabulation method adopted in the present work lists the laminar state-relationship as a function of the instantaneous mixture fraction. This has the advantage of easy incorporation of additional thermo-chemical quantities, i.e. species or laminar rates of formation. Also another important advantage of the tabulation method is that the convolution integral can be performed as accurately as required by simply adding additional rows in the table. Apart from the gaseous species there are the tar components which are largely emitted both as vapour and as liquid droplets of about 2-lOuxn diameter. High speed video shows under pyrolysis conditions that these species are converted to soot in a flame environment. Capturing particles on microscope slides showed jets of "tar" leaving the particle which had condensed in 7 |